For about the last fifty years, protective and decorative coatings based upon acrylic polymers have been increasingly used. These polymers have been utilized in many applications because they have provided a wide range of strength, flexibility, toughness, adhesion, degradation resistance and other film properties. Many acrylics, however, are solution polymers because they are prepared and applied as solutions of organic polymers,in organic solvents. In coatings technology, polymeric vehicles which included thermoplastic acrylic polymers required the use of organic solvents which often are toxic and/or subject to regulation that demand their reduction in coating compositions.
Environmental concern has become increasingly important. This concern not only extends to preservation of the environment for its own sake, but extends to safety for the public as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are often not only unpleasant, but contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to volatile organic compounds (VOCs) which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines relating to the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption by the states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and industrial coating industry which uses organic solvents which are emitted into the atmosphere.
The use of aqueous dispersions of polymeric vehicles, or alternatively, high solids polymeric vehicles are two general approaches that have been used to reduce VOCs in coating compositions. Aqueous systems have limited the molecular weights of the polymers used in the polymeric vehicle. This limited the hardness and other properties of the coating binders and films which resulted from the polymeric vehicles. Moreover, high levels of coalescents often have been required in aqueous thermoplastic vehicles. In water reducible systems with cross-linkers, as opposed to aqueous thermoplastic emulsions, molecular weights of polymers have been kept low and have required cross-linking.
The high solids approach often includes organic solvents or powder coatings. High solids, however, present problems in disposition of the polymeric vehicle. In powder systems requiring heat, disposition efficiency often is less and use of more than one color is difficult. Other high solids systems also may require specialized equipment because of the high solids content and the use of at least some organic solvents which are VOCs.
This invention is directed to aqueous systems which comprise aqueous dispersions of ammonia or amine salts of acrylic polymers which dispersions are low in VOCs and have less than about two weight percent organic solvent. Heretofore as described in the parent application to this application, dispersions of amine salts of high molecular weight acrylic polymers can be made by making the amine salt in a water immiscible solvent, mixing the acrylic polymer salt, water and solvent to form a salt/solvent/water mixture, inverting the mixture to form a solvent in water azeotrope and heating the azeotrope to remove the organic solvent and to provide a low VOC aqueous dispersion of the salt of the acrylic polymer having less than about 2 weight percent of organic solvent. In the latter process, generally water was added to the salified acrylic polymer/organic solvent combination. While the latter process and the dispersion made in the process are unique and advantageously provide extremely useful dispersions which may be used to provide polymeric vehicles and formulated coating compositions, certain problems exist for the process and compositions made by the process. Adding water to the acrylic polymer salt/solvent combination and heating the combination to invert a water in oil to an oil in water emulsion resulted in an undesirable increase in viscosity of the system during the creation of the azeotrope and removal of the organic solvent. This viscosity increase limits the molecular weight of the acrylic polymer salt for the aqueous dispersion. Further, when water is added to the acrylic polymer salt/solvent combination, and the resulting mixture is heated to invert the water in oil emulsion to an oil in water emulsion as well as to remove the organic solvent, there is a tendency for the heated azeotrope to unacceptably foam.
Using amine salts in the process pragmatically limits products to polymeric vehicles and formulated coating compositions which are baked or thermoset. Amine salts of acrylic polymers generally are not suitable for polymeric vehicles and formulated coatings which are to be air dried at ambient temperatures into a film or coating binder. Air drying at ambient temperatures frequently leaves residual amines in the film. These residual amines frequently result in poor film performance and even may be deleterious to the substrate coated by the coating composition. At first blush, ammonia salts of the acrylic polymers would seem an easy answer to the problem of residual high boiling amines in the coating composition. These salts will not result in residual high boiling amines in an air dried coating binder. Ammonia salts of some acrylic polymers, however, will not readily disperse in organic solvents; and as a result, solids levels of these polymeric salts are low (e.g. about 15%). As a result, extraordinary amounts of organic solvent generally would be required to make ammonia salts of acrylic polymers for use in making aqueous dispersions which would be suitable for use in making coating binders, polymeric vehicles and formulated coating compositions. The use of large amounts of organic solvent is not only expensive, but inappropriate with keeping VOC emissions low in manufacturing. Further, as previously noted, many organic solvent systems foam excessively during heating and azeotroping.
It is an object of this invention to provide water dispersions of high molecular weight acrylic polymers which dispersions have low VOCs.
It also is an object of this invention to provide a process for making aqueous dispersions of high molecular weight acrylic polymers which process permits high molecular weight polymers without high processing viscosities, excessive process times and foaming.
It is a further object of this invention to provide a low VOC cross-linkable polymeric vehicle which includes a high molecular weight acrylic polymer which is cured at elevated temperatures; or a polymeric vehicle which may be used in a coating composition which is air dried at ambient temperature.
Still further objects and advantages of the invention will be found by reference to the following description.